A huge number of optical spectra have been measured in the ultraviolet, violet, visible, infrared and far-infrared regions. However transmittance and reflectance can only be measured by the use of a spectrometer, it is very difficult to measure the transmittance and reflectance with the same accuracy. Our STAR GEM (Scatter, Transmission and Absolute Reflection using a Geminated Ellipsoid Mirror) is the first realization not only to overcome the difficulty but also to make the absolute measurements of transmittance and reflectance at any incident angle. The STAR GEM is a new optical accessory and is designed so that it can be incorporated into commercial Fourier-transform infrared spectrometers. Although the STAR GEM is used for infrared spectral measurements, the measurement methods, design principles, and features are generally applicable to other wavelengths as well.
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